Haoyu Li

Three-dimensional extended nonlocal photopolymerization driven diffusion model. Part II. Photopolymerization and model development

In Part I of this paper [J. Opt. Soc. Am. B31, 2638 (201410.1364/JOSAB.31.002638JOBPDE0740-3224)], an absorption model is used to predict the dye concentration and light intensity distribution inside a photopolymer medium volume. These results are now used as inputs to a Runge–Kutta algorithm acting as a subgrid of the finite-difference time-domain (FDTD) method. In this way, a full 3D time-dependent nonlocal photopolymerization driven diffusion model is implemented. This enables a more accurate and physical description of the evolutions of the holographic grating. The validity of the proposed model is examined by applying it to fit experimental data for acrylamide/polyvinyl alcohol photopolymer material layers containing the photosensitizer erythrosine B. Material parameter values are estimated by numerically fitting the experimentally obtained refractive index modulation growth curves and angular scans.

Three-dimensional extended nonlocal photopolymerization driven diffusion model. Part I. Absorption

When holographic gratings are recorded in a photopolymer material layer, the spatial distributions of the photoreactions taking place lead to the formation of nonuniform gratings in depth. In an effort to study such effects, in this series of papers a three-dimensional (3D) nonlocal photopolymerization driven diffusion (NPDD) model is developed. In Part I, we focused on describing the photoinitiation mechanisms by introducing a 3D dye absorption model that more accurately and physically describes the processes taking place. Then, the values of physical parameters are extracted by numerically fitting experimentally obtained normalized transmittance growth curves for a range of layer thicknesses in an acrylamide/polyvinyl alcohol (AA/PVA) photopolymer material sensitized by Erythrosine B (EB). In Part II [J. Opt. Soc. Am. B31, 2648 (2014)10.1364/JOSAB.31.002648JOBPDE0740-3224], applying the results in Part I, the full 3D photophysical and photochemical evolutions are modeled. Then the resulting 3D NPDD model is validated experimentally.

Modeling the nonlinear photoabsorptive behavior during self-written waveguide formation in a photopolymer

Photopolymer materials can be used as recording media for self-written waveguides (SWWs) as they can exhibit a large refractive index change and high photo-sensitivity. In free radical photo-polymerization systems, the dyes, functioning as the photosensitizer, strongly influence the material properties. During photo-illumination the spatial and temporal evolution of the dye concentration is an important factor leading to nonlinear absorption. In this paper, based on an investigation of the photochemical mechanisms, we analyze the nonlinear photo-absorptive effect during the photo-initiation processes. The time varying exposing light distribution is calculated and used to iteratively estimate the evolving cross-sectional refractive index and loss coefficient values. The model enables a more accurate and physical description of the optically induced growth of SWWs in such systems. Then SWWs formed in dry acrylamide/polyvinyl alcohol (AA/PVA) based photopolymer samples, containing different initial dye concentrations, are experimentally examined. The nonlinear absorptive behavior is quantified by comparing the model predictions and the experimental results.

Comparison of a new photosensitizer with erythrosine B in an AA/PVA-based photopolymer material.

Dyes often act as the photoinitiator PI/photosensitizer PS in photopolymer materials and are therefore of significant interest. The properties of the PI/PS used strongly influences grating formation when the material layer is exposed holographically. In this paper, the ability of a recently synthesized dye, D_1, to sensitize an acrylamide/polyvinyl alcohol (AA/PVA) based photopolymer is examined, and the material performance is characterized using an extended nonlocal photopolymerization-driven diffusion model. Electron spin resonance spin-trapping (ESR-ST) experiments are also carried out to characterize the generation of the initiator/primary radical, R(•), during exposure. The results obtained are then compared with those for the corresponding situation when using a xanthene dye, i.e., erythrosine B, under the same experiment conditions. The results indicate that the nonlocal effect is greater when this new photosensitizer is used in the material. Analysis indicates that this is the case because of the dyes (D_1) weak absorptivity and the resulting slow rate of primary radical production.

Self-written waveguides in a dry acrylamide/polyvinyl alcohol photopolymer material

For the first time it is demonstrated that permanent optical waveguides can be self-written in a solid acrylamide/polyvinyl alcohol photopolymer material. The novel (to our knowledge) technique used to prepare the polymeric medium used is described. It is demonstrated that the resulting waveguides formed can be used to guide different wavelengths. A standard theoretical model is used to predict both the evolution of the light intensity distribution and the channel formation inside the material during the exposure. The experimental results and the numerical simulations are compared, and good agreement is obtained.

Volumetric Light-field Encryption at the Microscopic Scale

We report a light-field based method that allows the optical encryption of three-dimensional (3D) volumetric information at the microscopic scale in a single 2D light-field image. The system consists of a microlens array and an array of random phase/amplitude masks. The method utilizes a wave optics model to account for the dominant diffraction effect at this new scale, and the system point-spread function (PSF) serves as the key for encryption and decryption. We successfully developed and demonstrated a deconvolution algorithm to retrieve both spatially multiplexed discrete data and continuous volumetric data from 2D light-field images. Showing that the method is practical for data transmission and storage, we obtained a faithful reconstruction of the 3D volumetric information from a digital copy of the encrypted light-field image. The method represents a new level of optical encryption, paving the way for broad industrial and biomedical applications in processing and securing 3D data at the microscopic scale.

A Review of Hologram Storage and Self-Written Waveguides Formation in Photopolymer Media

Photopolymer materials have received a great deal of attention because they are inexpensive, self-processing materials that are extremely versatile, offering many advantages over more traditional materials. To achieve their full potential, there is significant value in understanding the photophysical and photochemical processes taking place within such materials. This paper includes a brief review of recent attempts to more fully understand what is needed to optimize the performance of photopolymer materials for Holographic Data Storage (HDS) and Self-Written Waveguides (SWWs) applications. Specifically, we aim to discuss the evolution of our understanding of what takes place inside these materials and what happens during photopolymerization process, with the objective of further improving the performance of such materials. Starting with a review of the photosensitizer absorptivity, a dye model combining the associated electromagnetics and photochemical kinetics is presented. Thereafter, the optimization of photopolymer materials for HDS and SWWs applications is reviewed. It is clear that many promising materials are being developed for the next generation optical applications media.

Analysis of the absorptive behavior of photopolymer materials. Part I. Theoretical modeling

Photopolymers have received a great deal of attention due to their broad range of applications. The variation of their absorptive behavior during exposure is pivotal to the study of such materials. A model combining the associated electromagnetics and photochemical kinetics is presented to describe these absorptive processes. Such a model is critical in describing both self-modulations during holographic recording and also self-focusing effects. To describe the photophysical and photochemical changes taking place, a modulated equivalent electrical conductivity is introduced. Temporal variations of the concentrations of dye, monomer, and polymer are then predicted using the modified nonlocal photopolymerization driven diffusion model. The numerical convergence of the model is examined. Comparisons between the predictions of the model and experimental results, for both acrylamide/polyvinyl alcohol and Phenanthrenequinone doped poly(methyl methacrylate) photopolymer materials, are presented and analyzed in Part II of this paper.

Beam self-cleanup by use of self-written waveguide generated by photopolymerization.

A novel method for multimode fiber (MMF) laser-beam cleanup is introduced based on the optically induced growth and interaction of self-written waveguides (SWWs) in a photopolymer material. Theoretically, it is predicted that when the light is introduced into a free-radical photopolymerizable system from a MMF, the incident multichannel and structured beam shape can be caused to merge to form a single-channel Gaussian-like beam under specific exposure and material conditions. Experimental validation was carried out using a dry acrylamide/polyvinyl alcohol (AA/PVA) photopolymer sample. This work opens the door to studies involving self-developing laser beam cleanup and also to possible applications in photonic telecommunication systems and integrated optical devices.

Analysis of the absorptive behavior of photopolymer materials. Part II. Experimental validation

In the first part of this paper, a model describing photopolymer materials, which incorporates both the physical electromagnetic and photochemical effects taking place, was developed. This model is now validated by applying it to fit experimental data for two different types of photopolymer materials. The first photopolymer material, acrylamide/polyvinyl alcohol, is studied when four photosensitizers are used, i.e. Erythrosine B, Eosin Y, Phloxine B and Rose Bengal. The second type of photopolymer material involves phenanthrenequinone in a polymethylmethacrylate matrix. Using our model, the values of physical parameters, are extracted by numerical fitting experimentally obtained normalized transmittance growth curves. Experimental data sets for different exposure intensities, dye concentrations, and exposure geometries are studied. The advantages of our approach are demonstrated and it is shown that the parameters proposed by us to quantify the absorptive behavior in our model are both physical and can be estimated.